Just resolve the issue so wanna post a note here for future reference for others

It turns out that machine public IP address is not reachable for some reason (a fact that probably concerns the public gateway) without designating a media relay outbound proxy the direct RTP could not really be establlished. Thus I changed the voxalot account back to the default (which takes mediarelay) and specify my ATA to use outbound proxy then call goes through.

For some reason, that public IP is traceable in a number of institutions from various countries yet it is not visible from my provider's network. I don't even know whose fault it is since it has been working for years without issue.